1. Field of the Invention
The present invention relates to a process for the polymerisation of vinyl chloride in aqueous suspension or microsuspension and to the polymers produced by such a process.
2. Description of the Prior Art
It is well known to produce vinyl chloride polymers by using an aqueous suspension polymerisation process in which an aqueous suspension of vinyl chloride, or vinyl chloride and a copolymerisable monomer, is polymerised using an organo-soluble free-radical-yielding initiator and a suspension agent (generally a protective colloid), the agitation (achieved e.g. with a paddle or helical stirrer) being such that a granular slurry is formed. It is likewise well-known to produce vinyl chloride polymers by using an aqueous microsuspension polymerisation in which an aqueous dispersion of vinyl chloride, or vinyl chloride and a copolymerisable monomer, in the presence of a surfactant (emulsifying agent and/or suspension agent) and an organo-soluble free radical-yielding initiator, is homogenised by subjecting it to a violent shearing action in, e.g. a colloid mill, a high speed pump or an ultrasonic device, and the resulting homogenised dispersion polymerised in a stirred reactor to yield a vinyl chloride polymer latex.
It is a desirable objective in such vinyl chloride polymerisations to have the facility of activating the organo-soluble free-radical-yielding initiator at will by means other than by thermal decomposition during the course of the polymerisation in order to achieve a significant increase in the rate of polymerisation. Such a facility has various advantages, one e.g. being the prevention of autoacceleration wherein the rate of the polymerisation reaction increases sharply towards the end of the polymerisation when most of the monomer has been converted to polymer.
In the case of aqueous microsuspension polymerisation it has been proposed that the problem of autoacceleration may be overcome to a certain extent by carrying out the polymerisation in the presence of an initially present seeding product (produced by aqueous microsuspension polymerisation) containing all the initiator necessary for the polymerisation. Such a process is described in British Pat. No. 170 963. In British Pat. No. 1 435 425 it is suggested that the effect on reducing autoacceleration by using the process of British Pat. No. 1 170 963 is limited in usefulness. In British Pat. No. 1 435 425 the problem of autoacceleration in aqueous microsuspension polymerisation is said to be effectively overcome by adopting a variant of the process of British Pat. No. 1 170 963 in which the initiator (contained in the seed polymer) is activated with a monomer-soluble metal complex, which is progressively introduced pre-formed, or progressively produced in-situ, throughout the polymerisation process, and is formed by reaction between a water-soluble metallic salt and a complexing agent--the complexing agent causing the metal to change from its water-soluble form to a form soluble in vinyl chloride. A variant of this technique for overcoming autoacceleration in aqueous microsuspension polymerisation is also described in European Patent Publication No. 0038634; however in the process claimed therein, the initiator is activated with a monomer-soluble metallic complex while in a "free" form, i.e. while not being contained in the particles of a pre-formed seeding product.
Unfortunately, the initiator activation technique described in the above-mentioned British Pat. No. 1 435 425 and European Patent Publication No. 0038634 does not appear to be applicable to ordinary aqueous suspension polymerisation. Thus, for example, I have found that if one employs the favoured activation system described and exemplified in these publications, viz the use of cupric sulphate as the water-soluble metallic salt and ascorbic acid as the complexing agent, virtually no or very little activation of the organo-soluble initiator takes place.
In East German Pat. No. 152 347 a process similar to that of European Patent Publication No. 0 038 634 (in some respects) is employed wherein an oil and/or water-soluble metallic salt in the presence of a peroxide-type initiator is intermittently or progressively treated with a reducing agent during vinyl chloride polymerisation to achieve an improved course of the reaction. As a water-soluble metallic salt, the patent exemplifies solely cupric sulphate, with ascorbic acid being used as a reducing agent. However, as mentioned above, such a system is not applicable to ordinary aqueous suspension polymerisation and East German Pat. No. 152 347 exemplifies this system only for microsuspension polymerisation. East German Pat. No. 152 347 does, however, exemplify the use of an oil-soluble metal salt (the copper salt of coconut oil fatty acid, which is principally copper laurate) with ascorbic acid as a continuously added reducing agent and employs this system to activate an ordinary suspension polymerisation.
East German Pat. No. 152 347 also mentions that it is known to initiate polymerisations using oil-soluble initiators by employing systems which contain chelate complexes. However such systems are said in the patent to be unsatisfactory from the commercial viewpoint because of the large quantities of constituents that are required leading to products of unacceptable quality and cost. Additionally, I am not aware of any disclosures concerning vinyl chloride polymerisation which employ a system comprising a chelate complex where the polymerisation initiator is progressively activated at will throughout the course of the polymerisation by chemical interaction involving the chelate complex.
I have now discovered a process whereby polymerisations in aqueous vinyl chloride suspension may be considerably and progressively activated using systems which employ certain chelate complexes, normally in a very small truly catalytic quantity, thereby allowing a significantly increased polymerisation rate to be achieved. The process is equally applicable to polymerisations in aqueous microsuspension.